Instead of changing the speeds and jumping heights (which would probably be complicated as ****), couldn't the older mercenaries have a drastic damage increase? They seem to do the same damage as they did in Acclaim BOTS, but people now have 2x the HP.
Edit for some random stats:
I just played a game in which Minitanks were hitting 300-500 and the Double-Blade merc was chaining 1,000+ x3. That just isn't fair :3.